1. Field of the Invention
Interferometric systems for measuring the axial length of the human eye.
2. Description of Related Art
Low-coherence interferometry (LCI) is a powerful non-contact measurement technique. It is used to interferometrically measure and characterize weak scattering signals using low-coherence light. It finds broad use in microscopy, sensing applications for quality control in semiconductor and other industries, and for medical applications such as Optical Coherence Tomography (OCT). Measurement systems using the principles of low-coherence interferometry are sold commercially. For example, the OPTIGAGE™ that is manufactured and sold by Lumetrics. Inc. of West Henrietta N.Y., is based on LCI and may be used to measure thicknesses of various multilayered materials, such as lens stacks, auto glass, polymer films, medical tubing and other objects.
There are generally two main approaches used in low-coherence interferometry: Spectral Domain Interferometry (SDI), and Time Domain (TDI) Interferometry. The two approaches are different in speed, sensitivity, and measurement range. SDI has superior sensitivity and speed; however, it also has small measurement range. It is mainly used in optical coherence tomography, which requires image acquisitions at video-rates (on the order of 30 frames per second), and in general, does not involve large measurement ranges. TDI has a virtually unlimited dynamic range. However it is generally less sensitive and is much slower than SDI.
Non-invasive measurements of the axial eye length require large measurement range and thus cannot be achieved using SDI. While the TDI sensitivity is sufficient to detect the light reflected by the ocular interfaces, its speed is not sufficient to perform the axial length measurement of an eye. In general, it is difficult for a human subject to hold his eyes motionless for more than about 100 milliseconds. Thus the measurement should be made at a rate greater than 10 Hz, and preferably at least about 15 Hz.
Additionally, increased measurement speed is also associated with reduced sensitivity, which cannot be improved by simply increasing the output power of the light source, as the amount of the incident light is limited by safety regulations. The light source cannot be so powerful as to cause damage to the eye during a measurement cycle.
Accordingly, there remains a need for an inexpensive and robust method for measuring the axial length of a human eye, which has sufficient sensitivity to perform the length measurements with high speed and high precision.